The vegetation of the Parinacota Province (18 degrees 12 ' S, 69 degrees 08 ' W) is studied using the Zurich-Montpellier school of phytosociology methods. The study area is located in a mountainous high andean environment, between 3200 and 5300 in of elevation, under the influence of tropical xeric to pluviseasonal bioclimates, with summer rainfall. Sixty-six phytosociological releves were performed in the field and were analyzed by tabular comparison using additional already published releves. Four phytosociological classes include all zonal vegetation of the study area: Para-strephio lepidophyllae-Fabianetea densae, supratropical, orotropical and cryoro-tropical dwarf-scrubs and grasslands; Polylepidetea tarapacano-besseri, orotropical Polylepis forests; Anthochloo lepidulae-Dielsiochloetea floribundae, cryorotropical vegetation of rocky and solifluxional soils. Supratropical desert scrubs of the alliance Ambrosio artemisioldis-Atriplicion imbricatae, are included in the new order Atriplicetalia imbricatae of the class Opuntietea sphaericae. Azonal vegetation is included into the classes Plantagini rigidae-Distichietea muscoldis (high andean mires and bogs), Potamogetonetea (submerged vegetation) and Lemnetea minoris (floating plant communities). Fourteen associations were recognized, five of which are new (Oreocereo Ieucotrichi-Ambrosietum artemisioidis, Lophopappeturn tarapacani, Diplostephio meyenii-Fabianetum ramulosae, Wernerio aretioidis-Parastrephietum lucidae and Senecioni zoellneri-Azorelletum compactae) and one is proposed as provisional (Deyeuxio curvulae-Wernerietum incisae). One new alliance (Diplostephio meyenii-Fabianion ramulosae) and one provisional alliance (Senecionion zoellneri-scorzoneraefolii) are described, discussing their syntaxonomy on the basis of 1115 releves already published for the involved classes, including the releves published here. Four vegetation belts were recognized according to the altitudinal variation of the vegetation physiognomy, floristic composition and bioclimate. Climatic influences of the temperature and humidity on the altitudinal zonation of vegetation belts are discussed in the context of tropical high Andes. Two main floristic elements can be distinguished in the Parinacota Province: Pacific Desert and Puna. The first one is the more important component at low altitude, decreasing its participation in higher elevation habitats, where the second becomes dominant. The origin of these elements is discussed on the basis of paleoecological data: the Pacific Desert element seems to be mainly associated to the development of aridity in the western slope of the Andes since the Miocene, while the Puna element appears to be originated due to the raise of the Andes during the Plio-Pletstocene. The phytogeographical affinities of the Parinacota Province with the Mediterranean Andes and the Paramo are also discussed.
